1. Field of the Invention
The present invention relates to a system and method for transmitting signals, and more particularly, the present invention relates to a system and method for transmitting and receiving time sensitive, not fully reliable digital signals between a source and a receiver.
2. Discussion of the Related Art
Time sensitive, not fully reliable digital signals include single-media and multi-media data streams, including audio, audio and graphics, video, and synchronized audio and video data. As discussed herein, the concepts of the present invention are applicable to any system for streaming digital information from at least one sender to at least one receiver in which the data transmission is time sensitive but does not require the fidelity provided by full reliability. In addition, the data or information in the transmission may have a priority scheme, e.g., multiple priorities assigned to different portions, such as packets, of the data or information (e.g., heterogeneous priority).
For example, multimedia data are presented to a user in a time critical way. For example, the auditory experience of the user is hindered if the data are presented too slowly or in an incorrect order. If presented too slowly, the user may hear a lower frequency voice than belongs to the speaker, which decreases the fidelity of the presentation. The decreased fidelity diminishes the utility of such audio data as music. In the case of speech, if data are dropped out or swapped in sequence, the user may be unable to determine what the speaker is saying, which decreases the utility of the communication. As another example, the visual experience of the user is hindered if the video data is presented out of sequence or out of synchronization with the audio data. Out of sequence video data at time scales longer than transmission time for one frame causes smooth motion to become zigzagged as frames are shown out of sequence, destroying the utility for motion critical video such as dance, sporting events, and scientific research. Out of sequence video data on shorter time scales causes portions of a single frame to be presented at incorrect spatial positions on a display screen, so that the image is at best distorted or, at worst, unrecognizable.
Multi-media data takes many forms known in the art. For example, audio data is stored as files of binary data using various formats. In some formats, the data is compressed so that the number of binary digits (bits) when stored in the file is less than the number of bits used during presentation to a human observer. Example image formats, often indicated by extension on the names of the files used to store their data, include GIF, JPEG, TIFF, bit map (BMP), CGM, DXF, EPS, PCX, PDF, PIC, among others. Example audio formats, often indicated by extensions on the names of the files used to store their data, include waveform audio (WAV), MP3, audio interchange file format (AIFF), unix audio (AU), musical instrument digital interface (MIDI), and sound files (SND), among others. Example video formats, often indicated by extensions of the names of the files used to store their data, include QuickTime, AVI and the Motion Picture Experts Group format (MPEG), among others. Further treatment of the subject is provided in the book Video Communication, (1) Image and Video Compression Standards, V. Bhaskaran and K. Konstantinides, Kluwer Academic, 1995.
To allow a plurality of complex systems to communicate, a common set of standards has been established that system and component manufacturers have agreed to use in their systems and components. These standards relate to a basic set of functions. Among the functions, and at the most basic level, is the communication function and the rules, or protocols, for exchanging information. The applicable standard depends on the syntax of the data and the network or system over which the data is sent, for example, MPEG, Cable, Internet, etc. For example, two well-known Internet protocols are the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). The principles of the present invention are applicable to any such system, regardless of the underlying protocols.
TCP includes a feedback loop that allows for full reliability. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent. TCP attempts to recover from packet losses by allowing for multiple retransmissions of packets as indicated by the feedback information and adjusts the sending rate dynamically if it perceives packet losses. However, the trade-off for minimizing packet losses is an inherent delay caused by the retransmission process. Thus, TCP is particularly slow for multimedia transport; for example, video transmission is slow if TCP is used. In fact, systems using TCP under lossy conditions can “lock up” so that the user does not see or hear streaming information, thus compromising quality of service. The delay between sending a data packet from a server and receiving the packet at a client is called network latency. TCP is a good protocol for high-latency-tolerant data traffic requiring full reliability.
UDP has no reliability mechanisms, no feedback loop, and provides choppy video transmission. Because UDP is so unsophisticated (or “dumb”), UDP incurs no delay, but does not permit retransmission. UDP is better suited for communications that are not tolerant of high latency. For example, UDP is often used for broadcast transmission.
Applications have been adapted to improve the display quality of streaming signals transmitted over a network such as the Internet or Cable. However, such applications still do not provide high quality display because of the inherent problems in transmission when available bandwidth is limited.